This invention relates to a heat engine, and more particularly to the conversion of radiant heat, such as solar energy, into mechanical energy in an engine using special alloys which exist in two distinct crystallographic states with distinctly different mechanical properties. The heat engine of this invention is especially suited to solar energy conversion to mechanical energy in outer space.
A class of alloys, referred to as "shape memory alloys", exist in an austenitic state, characterized by an austenitic crystal structure, above a transition temperature range, and in a martentistic state, characterized by a martenistic crystal structure, below that temperature range. The transition temperature range is specific to the alloy composition and the temperature at which the alloy has been annealed.
In the austenitic state, such alloys exhibit high stiffness. In the martensitic state, on the other hand, they are relatively soft, pliable and easy to deform. When a piece of such an alloy is deformed in its martenistic state and then heated above the transition temeperature range to transform it into its austenitic state, it will "remember" and revert to its shape in its earlier austenitic state, i.e., prior to deformation in the martensitic state exerting considerable force against any restraining objects in doing so and being capable of generating mechanical energy thereby.
Alloys exhibiting "shape memory" include Ag-Cd-, Au-Cd, Au-Cu-Zn, Cu-Al-Ni, Cu-Zn, Ni-Ti, and certain steels. The most important alloy in this class from a practical standpoint is Ni-Ti in approximately 1:1 atomic ratio (53-57 weight percent Ni, 47-43 weight percent Ti) which is referred to as Nitinol, and whose preferred transition temeperature range is from 20 to 60 degrees Celsius.